Is a 115ml bottle of saline same as a bomb?

By Bruce Schneier / THE GUARDIAN , MINNEAPOLIS, MINNESOTA

Airport security found a bottle of saline in my luggage at Heathrow Airport last month. It was a 115ml bottle, slightly above the 100ml limit. Airport security in the US lets me through with it all the time, but UK security was stricter. The official confiscated it because allowing it on the airplane with me would have been too dangerous. And to demonstrate how dangerous he really thought that bottle was, he blithely tossed it in a nearby bin of similar liquid bottles and sent me on my way.

In a sense, this is a dumb game anyway; it’s an overly specific reaction to tactics instead of threats. We take away guns and bombs, so the terrorists use box cutters. We confiscate box cutters and corkscrews, so they put explosives in their sneakers. We screen footwear, so they use liquids. We take away liquids, and they’re going to do something else.

So why are we even playing? We’re playing because it’s politically impossible not to defend against the particular tactic the terrorists tried last year. But because we know that liquids aren’t really dangerous, we don’t treat them as such — which makes confiscating them completely ineffective.

There are two classes of contraband at airport security checkpoints: the class that will get you in trouble if you try to bring it on an airplane, and the class that will cheerily be taken away from you if you try to bring it on an airplane.

This difference is important; making security screeners confiscate anything from that second class is a waste of time and doesn’t make us any safer.

Let me explain. If you’re caught at airport security with a bomb or a gun, the screeners aren’t just going to take them away from you. They’re going to call the police and you’re going to be stuck for a few hours in an isolated room answering a lot of awkward questions. You may be arrested, and you’ll almost certainly miss your flight. At best, you’re going to have a very unpleasant day.

This is why articles about how screeners don’t catch every gun and bomb that goes through the checkpoints — and their track record is actually pretty mediocre — don’t bother me. Perfection is impossible, and trying would be too expensive. We can’t keep weapons out of prisons, how can we possibly keep them out of airports?

But the screeners don’t have to be perfect; they just have to be good enough. No terrorist is going to base his plot on getting a gun through airport security if there’s a decent chance of getting caught, because the consequences of getting caught are too severe.

Contrast that with a terrorist plot that requires a 350ml bottle of liquid. There’s no evidence that the 2006 liquid bombers actually had a workable plot, but assume for the moment that they did. If some copycat terrorists try to bring their liquid bomb through airport security and the screeners catch them — like they caught me with my opaque bottle labeled as “saline” — the terrorists can simply try again. In fact, they can try again and again. They can keep trying until they succeed. Because there are no consequences to trying and failing, the screeners have to be 100 percent effective. Even if the screeners slip up only once every hundred times, the plot can succeed.

The same is true for knitting needles, pocket knives, scissors, corkscrews, cigarette lighters and whatever else the airport screeners are confiscating this week. If there’s no consequence to getting caught with it, then confiscating it only hurts innocent people. At best, it mildly annoys the terrorists.